The present invention relates generally to magnetic resonance imaging (MRI), and more particularly, to a system and method to dissipate heat generated by predicting thermal generation based on a selected excitation of a coil assembly in an MRI apparatus and thereby maintain coil assembly temperature within acceptable operating limits.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (GxGy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
During patient scans, the gradient coils that produce the magnetic field gradients dissipate large amounts of heat, typically on the order of tens of kilowatts. The majority of this heat is generated by resistive heating of the copper electrical conductors that form the x, y, and z-axis gradient coils when these coils are energized. The amount of heat generated is in direct proportion to the electrical power supplied to the gradient coils. The large power dissipations not only result in increased temperature to the gradient coil, the heat produced is distributed within the gradient coil assembly, or resonance modules, and influences the temperature in two other critical regions. These two regions are located at the boundaries of the gradient assembly and include the patient bore surface and the warm bore surface adjacent to the cryostat that houses the magnets. Each of these three regions has specific maximum temperature limitations. In the resonance module, there are material temperature limitations, such as the glass transition temperature. That is, although the copper and fiber-reinforced backing of the coils can tolerate temperatures in excess of 120xc2x0 C., the epoxy to bond the layers typically has a much lower maximum working temperature of approximately 70-100xc2x0 C. On the patient bore surface, regulatory limits mandate a peak temperature on the patient bore surface of 41xc2x0 C. The warm bore surface also has a maximum temperature that is limited to approximately 40xc2x0 C. to prevent excessive heat transference through the warm bore surface and into the cryostat. Further, temperature changes of more than 20xc2x0 C. can cause field homogeneity variations due to a temperature dependence of the field shim material that exhibits a magnetic property variation with temperature.
Typically, the heat produced by the gradient coils in the resonance modules is removed from the gradient assembly by liquid filled cooling tubes embedded in the resonance modules at a given distance from the heat conductors. A liquid coolant, such as water, ethylene, or a propylene glycol mixture, enters the resonance module at a fixed temperature and flow rate, absorbs heat from the gradient coils as it is pumped through the cooling tubes, and transports the heat to a remote heat exchanger/water chiller. Heat is then ejected to the atmosphere by way of the heat exchanger/chiller. For each degree reduction of the coolant temperature as it enters the resonance module, the peak temperatures at each of the three critical regions (resonance module interior, patient bore surface, and warm bore surface) are also lowered.
However, in current systems, the minimum temperature of the coolant supplied to the resonance modules is limited by the dew point temperature of the ambient air. That is, since it is necessary to prevent the water vapor in the air from condensing in the resonance modules in general, and on the gradient coils in particular, the temperature of the coolant must remain above the dew point temperature of the ambient air. The high voltages and currents that are applied to the gradient coils dictates an atmosphere that must be free of such condensation. Current environmental specifications for MR rooms require 75% relative humidity at 21xc2x0 C., which requires a dew point temperature of 16xc2x0 C. Therefore, the minimum coolant temperature must be above 16xc2x0 C. under these conditions.
The maximum power which can be supplied to a resonance module is therefore limited by the external dew point temperature. To increase the power which can be received by the resonance module, it is necessary to lower the minimum coolant temperature. However, as indicated previously, environmental specifications limit the minimum coolant temperature to above 16xc2x0 C. for an MR room with 75% relative humidity at 21xc2x0 C. As a result, these current systems are unable to accommodate higher power patient scan sequences often required by resonance modules.
In these known systems, the lowest permissible coolant temperature is dictated by atmospheric conditions or the ambient dew point temperature. With these systems, the coolant temperature is set above the worst case dew point temperature based upon the given temperature and relative humidity specifications in the room housing the MR system.
Further, these systems must be kept from overheating. In case of increased temperatures of the resonance module or the patient surface, imaging scans must be interrupted or limited to low power sequences, which in turn reduces the efficiency and efficacy of the MR system. Time is then lost because imaging sessions cannot begin anew until the resonance module or patient surface cools sufficiently.
It would therefore be desirable to design a method and system to maintain gradient coil temperature within a specified range regardless of the selected excitation applied, thereby enabling higher power applications for faster imaging with improved image quality and longer scan times.
The present invention provides a predictive system and method overcoming the aforementioned drawbacks by removing heat from the gradient coil module of an imaging device based on the type of excitation applied while maintaining internal and external temperatures below maximum operating limits. Such a technique allows higher power applications for faster imaging with improved image quality, as well as, allowing longer scan times for interventional procedures.
A cooling system is provided to dissipate heat from an MRI resonance module. The cooling system includes a vacuum enclosure, a set of relative humidity, temperature and pressure sensors, and a control system that dynamically adjusts the temperature of coolant in cooling tubes embedded in the resonance module. The cooling fluid increases in temperature as it absorbs heat from the resonance module and transports the heat to a remote heat exchanger, such as, a water chiller. Since air and water vapor are removed from the vacuum enclosure containing the resonance module, condensation is prevented in the evacuated enclosure. As a result, the coolant temperature may be adjusted as needed to remove heat and maintain gradient coil temperatures within allowable levels.
Moreover, to further enhance proper operation and reliability, pressure and relative humidity sensors are placed in the vacuum enclosure to monitor for air and/or coolant leakage. To monitor condensation of water vapor on the exterior surfaces of the gradient coil, temperature sensors are installed on the patient and warm bore surfaces and in the vacuum enclosure. The control system is configured to provide the lowest practical coolant temperature while simultaneously preventing condensation on the patient and warm bore surfaces. Additionally, the relative humidity and pressure sensors may be used to trigger an alarm and disable the gradient coil drivers in response to an anomalous operating condition.
In accordance with one embodiment of the present invention, a method for cooling electrical coils in MRI device is provided. The method includes the step of determining a future thermal load for an electrical coil and determining an expected operating temperature range for the electrical coil based on the future thermal load. The expected operating temperature range is then compared to a device specification temperature range. If necessary, based on the comparison of the expected operating temperature range to the specification temperature range, cooling operating parameters are adjusted to drive the expected operating temperature range to be within the specification temperature range.
In another aspect of the present invention, a computer program is provided to maintain critical temperatures of an MRI system within acceptable limits. The computer program includes a set of instructions that when executed by a computer causes the computer to receive an imminent scan profile and predict a gradient thermal load required for the imminent scan profile. The set of instructions further causes the computer to determine a cooling profile to accommodate the gradient thermal load and to cool the MRI system according to the cooling profile.
In yet a further aspect of the present invention, an MRI apparatus is provided and includes a magnetic resonance imaging (MRI) system having a plurality of gradient coils. The gradient coils are configured to be positioned about a bore of a magnet to impress a polarizing magnetic field. The MRI system further includes an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images. The MRI apparatus also includes an input device to receive a selected scan sequence and a cooling system to dissipate heat from the plurality of gradient coils. The cooling system includes a temperature sensor positioned to sense an indication of gradient coil temperature and a set of coolant tubes having a coolant pass therethrough and in thermal contact to transfer heat from the gradient coils of the-MR system. A heat exchanger is connected to the coolant tubes to remove heat from the coolant. A control is connected to receive signals from the temperature sensor and send signals to the heat exchanger to control coolant temperature in response to the selected scan sequence.
In accordance with another aspect of the present invention, a predictive thermal control of an imaging device is provided. The thermal control includes an input to receive a user selected scan sequence and at least one temperature sensor positioned to sense an indication of gradient coil temperature of a gradient coil assembly of an imaging device. The control further includes a pressure sensor positioned to sense pressure of a vacuum enclosure housing the gradient coil assembly therein. A humidity sensor is also provided and positioned to sense relative humidity within the vacuum enclosure. The aforementioned sensors continuously provide feedback to a processor programmed to maintain device-operating temperature within a specification range, the processor is further programmed to determine a cooling profile based on the user selected scan sequence and, in response to the feedback, adjust the coolant profile on-the-fly to regulate proper cooling of the gradient coil assembly.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.